Control Methods and Key Technical Points for Lemon Gummosis
Lemon is rich in amino acids, vitamins, various trace elements and other nutrients essential to the human body, making it an excellent health fruit for human consumption. Lemon has a wide range of practical value, and demand is growing ever larger, with broad market prospects; at present, lemon is a fruit in short supply on the international market, with enormous commercialization potential.
Gummosis is a disease to which lemon is highly susceptible; it not only affects plant growth and development and reduces fruit yield, but also lowers lemon fruit quality, seriously affecting lemon production and the enthusiasm of fruit growers. In recent years, the disease has occurred commonly in all producing areas, with the diseased-plant rate in some orchards reaching more than 90%, which has attracted great attention.
1 Field symptoms
Gummosis is a common and frequently occurring disease in lemon production. It is known by different names depending on the site and period of occurrence: when it occurs on the trunk it is called gummosis or resin disease; when it damages leaves and green fruit in the field it is called sandpaper disease (melanose); and when it damages ripe fruit during storage it is called stem-end rot. Based on the phenomena of recurrent infection and self-healing of lesions in the field, the symptoms can be divided into two major categories: lesions with no obvious wound and lesions with obvious wounds; the no-obvious-wound lesions can further be divided into the gummosis-rot type and the gummosis-non-rot type. In the gummosis-rot type, the cortex of the diseased part rots, the duration is long, and the amount of gum exuded is large, making it the main type of field occurrence. The gumming site is mostly on the main trunk or at the forks of main branches; the cortex of the diseased part is water-soaked and exudes a translucent yellowish-brown gelatinous substance with a foul odor; the bark of the diseased part cracks and loosens and falls off, exposing the xylem, with raised scars appearing around it, and dense black small granules, namely pycnidia, growing on the surface or under the epidermis of the lesion. In the gummosis-non-rot type, the diseased part is reddish-brown, the cortex is dry and withered, slightly sunken, and does not peel off, with an orange-yellow or yellowish-brown band at the junction of diseased and healthy tissue, and gum exudation is not obvious.
2 Occurrence characteristics
According to surveys, lemon gummosis occurs commonly in orchards, mainly distributed on the main trunk and main branches and their forks, as well as on the sun-facing west and south branches and on windward parts prone to freezing injury. Orchards with higher N fertilizer application have widespread disease, and the more fruit a tree bears, the more severe the occurrence of gummosis. Gummosis can occur throughout the year, mostly after the tree begins to bear fruit; spring and autumn are suitable for the disease, with the most severe occurrence in the hot, humid months of April to May and August to September; older trees compared with younger trees, and plants with high leaf N content and low K/B and K/N ratios compared with plants with higher K content, have widespread and severe gummosis. Diseased fruit trees show yellowing and shedding of leaves, and tree vigor declines; in mild cases the tree sheds leaves and bears no fruit, and in severe cases the plant dies; orchards show leaf drop, dead branches and shrunken fruit, with serious reductions in yield and quality.
3 Control strategies
Gummosis seriously affects lemon yield and quality. Considerable research has been conducted on the pathogen, the pattern of disease occurrence and control measures for this disease, but the control effect is still not ideal, and there is currently no very effective control method in production. Therefore, according to the occurrence characteristics of gummosis, it is necessary to adopt corresponding preventive measures: first, implement nursery stock quarantine and disinfection; second, cultivate disease-free nursery stock; third, select and plant disease-resistant varieties; fourth, graft and replace branches to strengthen tree vigor; fifth, prune branches and clean the orchard to strengthen management; sixth, prevent freezing and control insects to reduce wounds; of course, exploring suitable management measures is also very important.
3.1 Sanitation and hygiene: do a good job of winter orchard cleaning, thoroughly prune diseased and dead branches and trunks, dig out and remove diseased and dead trees and burn them in a centralized manner, and treat the orchard with lime sulfur mixture or sodium pentachlorophenate for closing the orchard, to kill overwintering sources of inoculum; before the disease occurs in spring, remove and burn diseased, insect-damaged and dead branches and fallen leaves as early as possible to reduce sources of inoculum and improve the control effect; spray 3 degrees Baume lime sulfur mixture to control the spread and damage of the pathogen.
3.2 Agricultural control: do a good job of ditching and drainage in the orchard, improve ecological conditions, raise the level of fertilization, and earth up around the fruit trees to prevent freezing before winter temperatures drop. New orchards should be established on sites with good drainage and convenient irrigation, and rootstocks with strong resistance should be selected. Strengthen orchard field, fertilizer and water, and cultivation management, increase K and B content, and appropriately control N amount, which is conducive to controlling gummosis and improving fruit quality.
3.3 Physical control: insect and disease prevention, cold and freeze prevention, and infection prevention are key links in lemon orchard management: first, whitewash the trunk. Before winter begins, scrape off dead bark from bearing trees, remove mold layers, and apply a layer of whitewash to the main trunk and main branches, which can not only kill pathogens and various overwintering insect pests, but also enhance the tree's reflective ability, making the day-night temperature difference relatively stable and avoiding “sunburn by day and freezing by night”.
Second, inspect frequently and scrape and treat at any time. Once gummosis is found, use a sharp knife disinfected with alcohol to scrape off the diseased part until healthy tissue is reached; then apply 50% carbendazim or thiophanate 100-fold solution, or 1:3 alkaline water solution, to the diseased part, once a week for 2 consecutive times, to promote wound healing.
According to reports, wrapping with plastic film can cure gummosis. Moisten the gumming part of the branch or trunk, use a knife to pare off the outer skin of the affected part down to good phloem, then wrap tightly and completely with plastic film; in the peak growth period of the following summer, unwrap and rewrap once, and unwrap in the autumn of the third year; gummosis on large and small branches is all cured, with not a single recurrence. In addition, the flame scorching method can achieve a cure rate of up to 100%. Use a blowtorch aimed at the diseased part and burn directly from the outer edge toward the center for about 30 to 40 seconds, until the edge of the healthy part adjoining the rotten part no longer exudes purplish-brown gum.
In addition, using bud grafting on diseased branches, or cutting off the branch 10 to 20 cm above the gumming site and applying medicine for protection, while also doing a good job of catching and killing longhorn beetle adults in summer, can prevent and reduce the occurrence and damage of gummosis.
3.4 Chemical control: diseased orchards and trunks need chemical treatment, and the timing of application is mainly during the peak period of gummosis; the main control methods include spraying the canopy for protection, whitewashing the trunk to prevent insects, scraping gum and applying medicine at the diseased part, and spraying the orchard for control.
In addition, for severely diseased orchards, before the spring shoot sprouts, when two-thirds of the flowers have fallen, and at the young fruit stage, in combination with the control of other diseases, spray 50% Tuzet 500- to 800-fold solution, which can control infection of leaves and fruit; for mid- to late-maturing varieties, generally spray once in mid-August with 50% carbendazim 800-fold solution plus 1% borax; for gummosis-affected fruit that have already appeared, supplement with one spray of 1.2% borax plus 20 mg of 2,4-D, and they return to normal after 5 to 7 days. After fruit harvest, apply 50% thiophanate or carbendazim 200- to 500-fold solution plus 100 to 200 mg of 2,4-D to the stem end of the fruit, which gives good control of stem-end rot.